Crystalline semiconductor thin film, method of fabricating the same, semiconductor device and method of fabricating the same

ABSTRACT

There is provided a technique to form a single crystal semiconductor thin film or a substantially single crystal semiconductor thin film. An amorphous semiconductor thin film is irradiated with ultraviolet light or infrared light, to obtain a crystalline semiconductor thin film ( 102 ). Then, the crystalline semiconductor thin film ( 102 ) is subjected to a heat treatment at a temperature of 900 to 1200° C. in a reduced atmosphere. The surface of the crystalline semiconductor thin film is extremely flattened through this step, defects in crystal grains and crystal grain boundaries disappear, and the single crystal semiconductor thin film or substantially single crystal semiconductor thin film is obtained.

BACKGROUND OF THE INVENTION

[0001] 1. Field of the Invention

[0002] The present invention relates to a technique on a semiconductordevice using a semiconductor thin film, and particularly to asemiconductor device constituted by a thin film transistor (TFT) using acrystalline silicon film and a method of fabricating the same.

[0003] Incidentally, in the present specification, the term“semiconductor device” means any devices functioning by usingsemiconductor characteristics. Thus, the semiconductor device includesnot only a single semiconductor component such as a TFT, but also anelectrooptical device or semiconductor circuit including TFTs and anelectronic equipment having those.

[0004] 2. Description of Related Art

[0005] In recent years, a TFT used for an electrooptical device such asan active matrix type liquid crystal display device has been activelydeveloped.

[0006] The active matrix type liquid crystal display device is amonolithic display device in which a pixel matrix circuit and a drivercircuit are provided on the same substrate. Moreover, a system-on-panelhaving a built-in logic circuit such as a γ-correction circuit, a memorycircuit, and a clock generating circuit has been also developed.

[0007] Since such a driver circuit and a logic circuit are required toperform a high speed operation, it is unsuitable to use a noncrystallinesilicon film (amorphous silicon film) as an active layer. Thus, underthe present circumstances, a TFT using a crystalline silicon film(single crystal silicon film or polysilicon film) as an active layer hasbeen examined.

[0008] The present assignee has Japanese Patent Application Laid-openNo. Hei. 7-130652 disclosing a technique for obtaining a crystallinesilicon film on a glass substrate. The technique disclosed in thepublication is such that a catalytic element for facilitatingcrystallization is added into an amorphous silicon film, and a heattreatment is carried out to obtain a crystalline silicon film. Theentire disclosure of this patent is incorporated herein by reference.

[0009] According to this technique, it is possible to greatly lower thecrystallization temperature of the amorphous silicon film through theaction of the catalytic element by 50 to 100° C., and is also possibleto decrease a time required for crystallization down to ⅕to {fraction(1/10)}.

[0010] However, when circuit performance comparable to a conventionalLSI comes to be required for a circuit assembled with TFTs, suchcircumstances have occurred that it is difficult to fabricate a TFThaving satisfactory performance to meet the specification by using acrystalline silicon film formed with a conventional technique.

[0011] Incidentally, in the present specification, a semiconductor thinfilm having crystallinity, such as a single crystal semiconductor thinfilm, a polycrystalline semiconductor thin film, and a microcrystallinesemiconductor thin film, is generically referred to as a crystallinesemiconductor thin film.

SUMMARY OF THE INVENTION

[0012] An object of the present invention is to provide a technique forforming a crystalline semiconductor film to form a semiconductorcomponent having more excellent characteristics.

[0013] According to an aspect of the present invention, a method offabricating a crystalline semiconductor thin film is characterized bycomprising the steps of: carrying out a first heat treatment totransform an amorphous semiconductor thin film into a crystallinesemiconductor thin film by irradiating an ultraviolet light or infraredlight; and carrying out a second heat treatment for the crystallinesemiconductor thin film at 900 to 1200° C. in a reduced atmosphere.

[0014] In the above structure, the second heat treatment has to becarried out at such a temperature that a natural oxidation film (forexample, silicon oxide film) formed on the surface of the crystallinesemiconductor thin film can be reduced, and is specifically carried outin a temperature range of 900 to 1200° C. (preferably 1000 to 1100° C.).Besides, it is preferable that a treatment time is at least 3 minutes ormore, typically 3 minutes to 2 hours, and representatively 10 minutes to30 minutes. This is a time required to exhibit effects of the secondheat treatment.

[0015] Incidentally, the second heat treatment may be carried out afterthe crystalline semiconductor thin film is converted into island-likeportions. Besides, the heat treatment is carried out by furnaceannealing (annealing carried out in an electrothermal furnace).

[0016] The feature of the present invention is that a crystallinesemiconductor thin film is first formed by using a technique ofcrystallization by irradiating an ultraviolet light or infrared light(hereinafter referred to as laser crystallization), and the crystallinesemiconductor thin film is subjected to the heat treatment at 900 to1200° C. in the reduced atmosphere (typically, hydrogen atmosphere).

[0017] In this case, when ultraviolet light is used, it is appropriatethat excimer laser light or strong light emitted from an ultravioletlamp is used as a crystallization technique, and when infrared light isused, it is appropriate that strong light emitted from an ultravioletlaser or an infrared lamp is used.

[0018] As the excimer laser light, it is appropriate that KrF, XeCl, ArFor the like is used for an excitation gas. Further, as the infraredlight, Nd: YAG laser, Nd: glass laser, ruby laser or the like may beused.

[0019] The beam of the laser light may be formed to have a linear orplanar cross section. When the beam is a line-shaped beam, such a laserthat scans a laser light from one end of the substrate toward the otherend thereof is preferably used.

[0020] Further, when the beam is a planar beam, the area of aboutseveral tens cm² (preferably, 10 cm² or more) can be irradiated at onetime, and it is appropriate that a laser having a total output energy of5 J or more, preferably 10 J or more, is used. In this case, it ispreferable that the density of energy is 100 to 800 mJ/cm², and theoutput pulse width is 100 nsec or more, preferably 200 nsec to 1 msec.For the purpose of realizing the pulse width of 200 nsec to 1 msec, itis appropriate that a plurality of lasers are connected to one anotherand synchronization of the lasers are shifted to create a state whereplural pulses are mixed.

[0021] Incidentally, high temperature annealing in a reduced atmospherewhich is carried out for the crystalline semiconductor thin film thathas been crystallized has an effect to flatten the surface of thecrystalline semiconductor thin film. This is a result of enhancedsurface diffusion of semiconductor atoms to make the surface energyminimum.

[0022] The effect of flattening is very effective in the case where thecrystalline film is irradiated with excimer laser ultraviolet light.When irradiation of excimer laser is done, the semiconductor film isinstantaneously melted from its surface, and then, the meltedsemiconductor film is cooled and solidified from a substrate side byheat conduction to the substrate. In this solidifying step, the meltedsemiconductor film is recrystallized, and becomes a crystallinesemiconductor thin film with a large grain diameter. However, since thefilm is once melted, volume expansion occurs so that asperities (ridges)are produced on the surface of the semiconductor film. In the case of atop gate type TFT, since the surface having the asperities becomes aninterface to a gate insulating film, the component characteristics aregreatly affected.

BRIEF DESCRIPTION OF THE DRAWINGS

[0023] In the accompanying drawings:

[0024]FIGS. 1A to 1F are views showing fabricating steps of a thin filmtransistor;

[0025]FIGS. 2A to 2C are views showing the structures of electroopticaldevices;

[0026]FIGS. 3A to 3F and 4A to 4D are views showing the structure of anelectronic equipment;

[0027]FIG. 5 shows statistical data of bearing ratios at ½ of P-V;

[0028]FIG. 6 is a characteristic view of a thresholdlessantiferroelectric mixed liquid crystal;

[0029]FIG. 7 is a SEM observation photograph of the surface of acrystalline silicon film before high temperature annealing;

[0030]FIG. 8 is a SEM observation photograph of the surface of thecrystalline silicon film after high temperature annealing;

[0031]FIG. 9 is an AFM image of the surface of a crystalline siliconfilm before high temperature annealing;

[0032]FIG. 10 is a AFM image of the surface of the crystalline siliconfilm after high temperature annealing;

[0033]FIG. 11 is a histogram distribution and a bearing ratio curve ofthe height of an AFM image before high temperature annealing;

[0034]FIG. 12 is a histogram distribution and a bearing ratio curve ofthe height of the AFM image after high temperature annealing; and

DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENTS

[0035] First, by using experimental results obtained by the presentinventor, an effect of high temperature annealing of the presentinvention will be described.

[0036] An experimental procedure will first be explained. An amorphoussilicon film with a thickness of 50 nm was formed on a quartz substrate.A low pressure CVD method was used for film formation, and disilane(Si₂H₆) (flow rate: 250 sccm) and helium (He) (flow rate: 300 sccm) wereused as film forming gases. The temperature of the substrate was made465° C. and the pressure at film formation was made 0.5 torr.

[0037] The surface of the amorphous silicon film was etched by bufferedhydrofluoric acid to remove a natural oxidation film and pollution.Next, the amorphous silicon film was irradiated with XeCl excimer laserlight to be crystallized. An atmosphere of the laser irradiation was theair, and the substrate temperature was room temperature, the density oflaser energy was 400 mj/cm², and the pulse width of the laser light was150 nsec.

[0038] Then the crystalline silicon film was subjected to a hightemperature annealing treatment. The condition of the high temperatureannealing treatment was made as follows: An atmosphere was made hydrogenof 100%, the degree of vacuum was 700 torr, annealing temperature was1000° C., and an annealing time was 25 minutes. Incidentally, before thehigh temperature annealing treatment, the crystalline silicon film wassubjected to a wet etching treatment by hydrofluoric acid, so that anatural oxidation film and pollution on the surface were removed.

[0039] For the purpose of ascertaining the effect of the hightemperature annealing, the surface of the crystalline silicon filmbefore and after the high temperature annealing was observed by SEM.FIG. 7 shows an observation photograph before the high temperatureannealing, and FIG. 8 shows an observation photograph after the hightemperature annealing. As is apparent from FIGS. 7 and 8, the surfaceshapes are clearly different before and after the high temperatureannealing.

[0040] Further, the surface shape of the silicon film was also observedby an AFM (Atomic Force Microscope). FIG. 9 shows an observation imageof the crystalline silicon film by the AFM before the high temperatureannealing, and FIG. 10 shows an observation image of the crystallinesilicon film by the AFM after the high temperature annealing.Incidentally, the range of observation is a rectangular region of 1.5μm×1.5 μm in both FIGS. 9 and 10.

[0041] As is apparent from FIGS. 9 and 10, the surface shapes of thecrystalline silicon film before and after the high temperature annealingare clearly different. Although asperities exist on the surface of thecrystalline silicon film before and after the high temperatureannealing, before the high temperature annealing, a protrusion is steep,and its top portion is sharp, and the surface totally shows a serrateshape. When the surface having such protrusions becomes an interfacebetween a gate insulating film and a channel formation region, it isthinkable that the component characteristics suffer a very badinfluence. On the contrary, a protrusion after the high temperatureannealing is smooth, and its top portion is round, so that thecharacteristics of the interface between the gate insulating film andthe channel formation region are improved as compared with those beforethe high temperature annealing.

[0042] Although it is understood that the surface of the crystallinesilicon film is flattened and smoothed even from the observation imagesshown in FIGS. 7 to 10, a histogram distribution of heights of AFMimages was calculated so as to further quantify the difference of thesurface shapes before and after the high temperature annealing. Further,a bearing ratio curve of the histogram distribution was calculated. Thebearing ratio curve is a curve expressing a cumulative frequency of thehistogram distribution.

[0043]FIGS. 11 and 12 show the histogram of the heights of the AFMimages and the bearing ratio curve. FIG. 11 shows data before the hightemperature annealing, and a pitch of the histogram is about 0.16 nm.FIG. 12 shows data after the high temperature annealing, and a pitch ofthe histogram is about 0.20 nm.

[0044] The measurement region by the AFM is 1.5 μm×1.5 μm. The bearingratio curve is a curve expressing the cumulative frequency of data ofthe histogram. The curves of FIGS. 11 and 12 are obtained throughaccumulation from the maximum value of the height, and expresses anoccupation ratio (%) of areas with height from the maximum value to anarbitrary value to the total area. In FIGS. 11 and 12, the horizontalline shown by a dotted line in the graph indicates the value of ½ of theP-V value (Peak to Valley, difference between the maximum value and theminimum value in height).

[0045] Further, in the silicon film before and after the hightemperature annealing, the AFM images were observed in ten regions(rectangle region of 1.5 μm×1.5 μm), and the beaming ratios at 2⁻¹ (P-Vvalue) in the respective observation regions were calculated. FIG. 5shows the bearing ratios in the respective observation regions and theirstatistical data.

[0046] When the curves in FIGS. 11 and 12 are compared with each other,although the height distribution before the high temperature annealingis inclined toward a low portion side, the inclination is shifted towarda high portion side after the high temperature annealing, and thehistogram is symmetrical with respect to the position of ½ of the P-V.This can be easily understood from the bearing ratio curve.

[0047] The bearing ratio at the height of 2⁻¹(P-V) is about 20% in FIG.11, and about 51% in FIG. 12. That is, an occupation ratio of an area ofa region where the height is within the range from the maximum value to2⁻¹(P-V value) to the total area is about 20% before the hightemperature annealing, and about 51% after the high temperatureannealing. From the difference in this ratio as well, it can beunderstood that the sharp top portion has been rounded and the surfaceof the silicon film has been flattened by the high temperatureannealing.

[0048] In the present invention, the surface shape of the crystallinesilicon film is quantified by the bearing ratio at 2⁻¹(P-V value), andfrom experimental results, it is presumed that the bearing ratio at 2⁻¹(P-V value), that is, in a predetermined observation region, anoccupation ratio of a region where the height exists in the range fromthe maximum value to 2⁻¹(P-V value) is within the range of 6 to 28% inthe film before the high temperature annealing, and 29 to 72% in thefilm after the high temperature annealing.

[0049] Incidentally, the range of the bearing ratio is set from thestatistical data of FIG. 5, and is a value calculated from an averagevalue ±3σ of the bearing ratio at 2⁻¹ (P-V value) . The bearing ratio isa value accumulated from the maximum value of the height.

[0050] As described above, since the crystalline semiconductor thin filmcrystallized by ultraviolet light such as excimer laser light iscrystallized after the surface has been melted, the occupying ratio of aregion where the height is within the range from the maximum value to ½of the difference between the maximum value and the minimum value is 6to 28% to a predetermined region. In the present invention, since thiscrystalline semiconductor thin film is subjected to the high temperatureannealing, the occupation ratio of this region is changed to 29 to 72%,and the top portion of the protrusion of the film surface can be madesmooth.

[0051] This step has also an effect to greatly decrease defects existingin crystal grains and crystal grain boundaries. This effect is obtainedthrough a terminating effect of uncombined bonds by hydrogen, a removingeffect of impurities by hydrogen, and recombination of semiconductoratoms with the effect. Thus, for the purpose of causing these effects tobe effectively exhibited, the treatment time as set forth above becomesnecessary.

[0052] Thus, it is necessary to carry out the heat treatment step in thereduced atmosphere by furnace annealing. If the heat treatment iscarried out by irradiation of ultraviolet light or infrared light,recrystallization progresses in a nonequilibrium state so thatcontinuity of crystal lattices at crystal grain boundaries is damaged,which is not preferable. In this point, in the furnace annealing, sincerecrystallization progresses in an equilibrium state, such a problem canbe avoided.

[0053] According to another aspect of the present invention, the methodof the present invention is characterized by comprising the steps of:

[0054] forming an amorphous semiconductor thin film on a substratehaving an insulating surface;

[0055] carrying out a first heat treatment to transform the amorphoussemiconductor thin film into a crystalline semiconductor thin film byirradiating ultraviolet light or infrared light;

[0056] carrying out a second heat treatment for the crystallinesemiconductor thin film in a reduced atmosphere including a halogenelement; and

[0057] prior to the step of forming the amorphous semiconductor thinfilm, adding a catalytic element for facilitating crystallization of theamorphous semiconductor thin film to the substrate having the insulatingsurface.

[0058] In such an arrangement, the second heat treatment is carried outat a temperature range of 900 to 1200° C. This step aims at a getteringfunction of the halogen element to a metal element, and has an object toremove as halogen compound the metal element existing in a crystallinesemiconductor thin film.

[0059] In the following, preferred embodiments of the present inventionwill be described in detail.

[0060] Embodiment 1

[0061] In this embodiment, the steps of fabricating a TFT on a substratein accordance with the present invention will be described, Adescription will be made with reference to FIG. 1.

[0062] First, a quartz substrate was prepared as a substrate 101. Amaterial having high heat resistance must be selected as the substrate101. Instead of the quartz substrate, a substrate of a material havinghigh heat resistance, such as a silicon substrate, a ceramic substrate,or a crystallized glass substrate, may be used.

[0063] However, although an under film may not be provided in the casewhere the quartz substrate is used, it is preferable to provide aninsulating film as the under film in the case where other materials areused. As an insulating film, it is appropriate that either one of asilicon oxide film (SiOx), a silicon nitride film (SixNy), a siliconnitride oxide film (SiOxNy), and an aluminum nitride film (AlxNy), or alaminate film of those is used.

[0064] Besides, it is effective to use an under film laminate arefractory metal layer and a silicon oxide film since a heat radiationeffect is greatly increased. Even the laminate structure of theforegoing aluminum nitride film and the silicon oxide film exhibits asufficient heat radiation effect.

[0065] After the substrate 101 having the insulating surface wasprepared in this way, a crystalline silicon film with a thickness of 30nm was formed by using a crystallization technique using a excimerlaser. Only the outline will be described in the present embodiment.

[0066] For the purpose of forming an amorphous silicon film, in thisembodiment, disilane (Si₂H₆) was used as a film forming gas. Anamorphous silicon film (not shown) with a thickness of 20 to 60 nm wasformed by a low pressure CVD method. At this time, it is important tothoroughly control the concentration of impurities, such as C (carbon),N (nitrogen), and O (oxygen) mixed in the film. This is because if theamount of these impurities is high, the progress of crystallization isprevented.

[0067] The applicant controlled the impurity concentration so that theconcentration of carbon and nitrogen became 5×10¹⁸ atoms/cm³ or less(preferably 1×10¹⁸ atoms/cm³ or less, more preferably, 5×10¹⁷ atoms/cm³or less, further more preferably 2×10¹⁷ toms/cm³), the concentration ofoxygen became 1.5×10¹⁹ atoms/cm³ or less (preferably 5×10¹⁸ atoms/cm³ orless, more preferably 1×10¹⁸ atoms/cm³). Further, control was made sothat the concentration of metal elements became 1×10¹⁷ atoms/cm³ orless. When such control of concentration has been made at a filmformation stage, if only external pollution is prevented, impurityconcentration is not increased during the steps of fabricating a TFT.

[0068] After the amorphous silicon film was formed, dehydrogenation wascarried out for about one hour at 450° C., and thereafter the step ofcrystallizing the amorphous silicon film was carried out using XeClexcimer laser light excited by the X-ray (second heat treatment). Inthis embodiment, the area of the laser irradiation was 10 cm×10 cm, thedensity of laser energy was 300 mJ/cm², and the pulse width of the laserlight was 150 nsec. A crystalline silicon film 102 was thus obtained(FIG. 1A).

[0069] Incidentally, if film quality equal to the amorphous silicon filmformed by the reduced pressure CVD method may be obtained, a plasma CVDmethod may be used. Instead of the amorphous silicon film, an amorphoussemiconductor thin film such as a film of silicon germanium (expressedby Si_(X)Ge_(1−x) (0<X<1)) in which germanium is contained in anamorphous silicon film may be used. In that case, it is desirable thatgermanium contained in silicon germanium is made 5 atomic% or less.

[0070] Next, a heat treatment within a temperature range of 900 to 1200°C. (preferably 1000 to 1150° C.) was carried out in a reducedatmosphere. In this embodiment, a heat treatment at 1050° C. for 20minutes was carried out in a hydrogen atmosphere (FIG. 1C). As a result,the occupying ratio of a region where the height is within the rangefrom the maximum value to ½ of the difference between the maximum valueand the minimum value can be 29 to 72% (FIG. 1B).

[0071] As the reduced atmosphere, although a hydrogen atmosphere, anammonia atmosphere, or an inert gas atmosphere containing hydrogen orammonia (mixture atmosphere of hydrogen and nitrogen or hydrogen andargon) is desirable, flattening of the surface of the crystallinesilicon film can be made by even the inert gas atmosphere. However, ifreduction of a natural oxidation film is carried out by using a reducingfunction, a number of silicon atoms with high energy are produced andthe flattening effect is consequently raised, so that the reducedatmosphere is preferable.

[0072] However, attention must be paid especially to a point that theconcentration of oxygen or oxygen compound (for example, OH group)contained in the atmosphere is made 10 ppm or less (preferably 1 ppm orless). Otherwise, the reducing reaction by hydrogen may not occursufficiently.

[0073] In this way, a crystalline silicon film 103 was obtained. Thesurface of the crystalline silicon film 103 was greatly flattened by ahydrogen heat treatment at a high temperature such as 900 to 1200° C.Besides, since the heat treatment was carried out at a high temperature,lamination defects and the like hardly existed in the crystal grains.

[0074] After the crystalline silicon film 103 was obtained in this way,the crystalline silicon film 103 was next patterned to form an activelayer 104. In this embodiment, although the heat treatment in thehydrogen atmosphere is carried out before the active layer 111 isformed, the heat treatment may be carried out after the active layer isformed. In the case, it is preferable that since patterning has beenmade, stress generated in the crystalline silicon film is relieved.

[0075] Then a thermal oxidation step was carried out so that a siliconoxide film 105 with a thickness of 10 nm was formed on the surface ofthe active layer 104. This silicon oxide film 105 functions as a gateinsulating film. Besides, since the film thickness of the active layer104 was decreased by 5 nm, the film thickness became 30 nm. In view ofthe film decrease by the thermal oxidation, it is necessary to determinethe film thickness of the amorphous silicon film (starting film) so thatan active layer 111 (especially a channel formation region) with athickness of 5 to 40 nm finally remains.

[0076] After the gate insulating film 105 was formed, a polycrystallinesilicon film having conductivity was formed thereon and a gate wiringline 106 was formed by patterning (FIG. 1C).

[0077] In this embodiment, although the polycrystalline silicon filmhaving N-type conductivity is used as the gate wiring line, a materialis not limited to this. Particularly, for the purpose of lowering theresistance of the gate wiring line, it is also effective to usetantalum, tantalum alloy, or laminate film of tantalum and tantalumnitride. Further, in order to attain a gate wiring line with lowresistance, it is also effective to use copper or copper alloy.

[0078] After the state of FIG. 1C was obtained, an impurity to giveN-type conductivity or P-type conductivity was added to form an impurityregion 107. The impurity concentration at this time was determined inview of an impurity concentration of a subsequent LDD region. In thisembodiment, although arsenic with a concentration in 1×10¹⁸ atoms/cm³was added, it is not necessary to limit the impurity and theconcentration to those of this embodiment.

[0079] Next, a thin silicon oxide film 108 with a thickness of about 5to 10 nm was formed on the surface of the gate wiring line 106. It isappropriate that this film is formed by using a thermal oxidation methodor a plasma oxidation method. The formation of this silicon oxide film108 has an object to cause the film to function as an etching stopper ina subsequent side wall forming step.

[0080] After the silicon oxide film 108 that functions as an etchingstopper was formed, a silicon nitride film was formed and etch back wascarried out, so that a side wall 109 was formed. In this way, the stateof FIG. 1D was obtained.

[0081] Incidentally, in this embodiment, although the silicon nitridefilm was used as the side wall, it is also possible to use apolycrystalline silicon film or an amorphous silicon film. Of course, itis needless to say that if a material of the gate wiring line ischanged, a material which can be used as the side wall is also changedaccording to that.

[0082] Next, an impurity with the same conductivity as that in theprevious step was again added. The concentration of the impurity addedat this time was made higher than that in the previous step. In thisembodiment, although arsenic is used as an impurity and itsconcentration is made 1×10²¹ atoms/cm³, it is not necessary to makelimitation to this. By the adding step of the impurity, a source region110, a drain region 111, an LDD region 112, and a channel formationregion 113 were defined (FIG. 1E).

[0083] After the respective impurity regions were formed in this way,activation of the impurity was carried out by a heat treatment such asfurnace annealing, laser annealing, or lamp annealing.

[0084] Next, silicon oxide films formed on the surfaces of the gatewiring line 106, the source region 110, and the drain region 111 wereremoved to expose the surfaces of those. Then a cobalt film (not shown)with a thickness of about 5 nm was formed and a heat treatment step wascarried out. A reaction of cobalt and silicon occurred by this heattreatment, so that a silicide layer (cobalt sillicide layer) 114 wasformed (FIG. 1F).

[0085] This technique is a well-known salicide technique. Thus, insteadof cobalt, titanium or tungsten may be used, and a heat treatmentcondition and the like may be determined by referring to a well-knowntechnique. In this embodiment, the heat treatment step was carried outby irradiation of infrared light.

[0086] After the silicide layer 114 was formed in this way, the cobaltfilm was removed. Thereafter, an interlayer insulating film 115 with athickness of 1 μm was formed. As the interlayer insulating film 115, itis appropriate that a silicon oxide film, a silicon nitride film, asilicon nitride oxide film, or a resin film (polyimide, acryl,polyamide, polyimidoamide, benzocyclobutene (BCB), etc.) is used. Theseinsulating films may be laminated in a free combination.

[0087] Next, contact holes were formed in the interlayer insulating film115, and a source wiring line 116 and a drain wiring line 117 made of amaterial containing aluminum as its main ingredient were formed.Finally, the whole component was subjected to furnace annealing at 300°C. for 2 hours in a hydrogen atmosphere, so that hydrogenating wascompleted.

[0088] A TFT as shown in FIG. 1F was obtained in this way. Incidentally,the structure explained in this embodiment is merely an example, and aTFT structure to which the present invention can be applied is notlimited to this. The present invention can be applied to a TFT of anywell-known structure. Besides, it is not necessary to limit numericalvalue conditions in steps subsequent to formation of the crystallinesilicon film 103 to those of this embodiment. Further, there is noproblem if a well-known channel doping step (impurity adding step forcontrolling a threshold voltage) is introduced to somewhere in thisembodiment.

[0089] Besides, in this embodiment, since the concentration ofimpurities such as C, N, and O was thoroughly controlled at the stage offilm formation of the amorphous silicon film as the starting film, theconcentration of each impurity contained in the active layer of thecompleted TFT was such that the concentration of carbon and nitrogenremained to be 5×10¹⁸ atoms/cm³ or less (preferably 1×10¹⁸ atoms/cm³ orless, more preferably 5×10¹⁷ /cm³ or less, further more preferably2×10¹⁷ cm⁻³ or less), and the concentration of oxygen remained to be1.5×10¹⁹ atoms/cm³ or less (preferably 5×10¹⁸ atoms/cm³ or less, morepreferably 1×10¹⁸ cm⁻³ or less). The concentration of metal elements was1×10¹⁷ atoms/cm³ or less.

[0090] Besides, the present invention can be applied to not only a topgate structure but also to a bottom gate structure typified by a reversestagger TFT.

[0091] Although the description has been made on the N-channel TFT as anexample, it is also easy to fabricate a P-channel TFT throughcombination with a well-known technique. Further, through combinationwith a well-known technique, it is also possible to form a CMOS circuitby fabricating an N-channel TFT and a P-channel TFT on the samesubstrate and by complementarily combining them.

[0092] Further, in the structure of FIG. 1F, if a pixel electrode (notshown) electrically connected to the drain wiring line 117 is formed bya well-known means, it is also easy to form a pixel switching element ofan active matrix type display device. U.S. Pat. No. 5,712,495 issued toSuzawa is an example for showing a structure of an active matrix device.An entire disclosure of this patent is incorporated herein by reference.It is also preferable to use an LDD TFT as a switching transistor for apixel.

[0093] The present invention can be also carried out when an activematrix type electrooptical device such as a liquid crystal displaydevice or an EL (electroluminescence) display device is fabricated.

[0094] Embodiment 2

[0095] In this embodiment, the case will be described where the step ofremoving a metal element in the film is used together with the step ofhydrogen annealing at 900-1200° C. according to the embodiment 1.

[0096] In this embodiment, a heat treatment was carried out at 900 to1200° C. in the atmosphere where 0.1 to 5 wt % of hydrogen halide(typically, hydrogen chloride) is combined in the hydrogen atmosphere.Besides, NF₃ or HBr may be used as hydrogen halide.

[0097] By adopting this embodiment, it is possible to remove or lower ametal element from the crystalline silicon film. Since the concentrationof the metal element is lowered down to 1×10¹⁷ atoms/cm³ or less, it ispossible to prevent TFT characteristics (especially off current value)from fluctuating by the existence of the metal element.

[0098] Embodiment 3

[0099] In this embodiment, an example of a reflection-type liquidcrystal display device fabricated according to the present invention isshown in FIGS. 2A to 2C. Since well-known means may be used for afabricating method of a pixel TFT (pixel switching element) and for acell assembling step, their detailed descriptions will be omitted.

[0100] In FIG. 2A, reference numeral 11 denotes a substrate (ceramicsubstrate provided with a silicon oxide film) having an insulatingsurface, 12 denotes a pixel matrix circuit, 13 denotes a source drivercircuit, 14 denotes a gate driver circuit, 15 denotes an oppositesubstrate, 16 denotes an FPC (Flexible Printed Circuit), and 17 denotesa signal processing circuit. As the signal processing circuit 17, acircuit for carrying out such processing that an IC has beensubstituted, such as a D/A converter, a γ-correction circuit, and asignal dividing circuit, can be formed. These circuits are desirablyconstituted with TFTs in accordance with the present invention. In analternative, it is also possible to provide an IC chip on a glasssubstrate and to carry out signal processing on the IC chip.

[0101] Moreover, although the description has been made of the liquidcrystal display device as an example, the present invention can also beapplied to an EL (electroluminescence) display device or an EC(electrochromic) display device as long as the device is an activematrix type display device.

[0102] Here, an example of a circuit constituting the driver circuits 13and 14 of FIG. 2A is shown in FIG. 2B. Since the TFT portion has beenexplained in the embodiment 1, only necessary portions will be describedhere.

[0103] In FIG. 2B, reference numerals 501 and 502 denote N-channel TFTs,and 503 denotes a P-channel TFT. The TFTs 501 and 503 constitute a CMOScircuit. Reference numeral 504 denotes an insulating layer made of alaminate film of a silicon nitride film/a silicon oxide film/a resinfilm. A titanium wiring line 505 is provided thereon, and the foregoingCMOS circuit and the TFT 502 are electrically connected. The titaniumwiring line is covered with an insulating layer 506 made of a resinfilm. The two insulating layers 504 and 506 have also a function as aflattened film.

[0104] A part of a circuit constituting the pixel matrix circuit 12 ofFIG. 2A is shown in FIG. 2C. In FIG. 2C, reference numeral 507 denotes apixel TFT made of an N-channel TFT of double gate structure, and a drainwiring line 508 is formed so as to widely extend in a pixel region.Incidentally, other than the double gate structure, a single gatestructure, a triple gate structure, or the like may be used.

[0105] An insulating layer 504 is provided thereon, and a titaniumwiring line 505 is provided thereon. At this time, a recess portion isformed in a part of the insulating layer 504, and only silicon nitrideand silicon oxide on the lowermost layer are made to remain. By this, anauxiliary capacitance is formed between the drain wiring line 508 andthe titanium wiring line 505.

[0106] The titanium wiring line 505 provided in the pixel matrix circuithas an electric field shielding effect between source/drain wiring linesand a subsequent pixel electrode. Further, it also functions as a blackmask at a gap between a plurality of pixel electrodes.

[0107] Then an insulating layer 506 is provided to cover the titaniumwiring line 505, and a pixel electrode 509 made of a reflectiveconductive film is formed thereon. Of course, contrivance for increasingreflectivity may be made to the surface of the pixel electrode 509.

[0108] By using the present invention, it is possible to fabricate thereflection-type liquid crystal display device having the structure asdescribed above. Of course, when a well-known technique is combined, atransmission-type liquid crystal display device can also be fabricated.Further, when a well-known technique is combined, an active matrix typeEL display device can also be easily fabricated.

[0109] Although not distinguished in the drawings, it is also possibleto make the film thicknesses of gate insulating films different betweenthe pixel TFT constituting the pixel matrix circuit and the CMOS circuitconstituting the driver circuit and the signal processing circuit.

[0110] In the pixel matrix circuit, since a driving voltage applied tothe TFT is high, the gate insulating film with a film thickness of 50 to200 nm is required. On the other hand, in the driver circuit and thesignal processing circuit, a driving voltage applied to the TFT is low,while high speed operation is required. Thus, it is effective to makethe film thickness of the gate insulating film about 3 to 30 nm, whichis thinner than that of the pixel TFT.

[0111] Embodiment 4

[0112] In the liquid crystal display device fabricated in the aboveembodiment, other than a TN liquid crystal, various liquid crystals maybe used. For example, it is possible to use a liquid crystal disclosedin “Characteristics and Driving Scheme of Polymer-Stabilized MonostableFLCD Exhibiting Fast Response Time and High Contrast Ratio withGray-Scale Capability” by H. Furue et al. 1998 SID, “A Full-ColorThresholdless Antiferroelectric LCD Exhibiting Wide Viewing Angle withFast Response Time” by T. Yoshida et al., 1997, SID DIGEST, 841,“Thresholdless antiferroelectricity in liquid crystals and itsapplication to displays” by S. lnui et al., 1996, J. Mater. Chem. 6(4),671-673, or U.S. Pat. No. 5,594,569.

[0113] A liquid crystal exhibiting antiferroelectricity in sometemperature range is called an antiferroelectric liquid crystal. Inmixed liquid crystals including antiferroelectric liquid crystals, thereis a thresholdless antiferroelectric mixed liquid crystal exhibitingelectrooptical response characteristics in which transmittance iscontinuously changed with respect to an electric field. Somethresholdless antiferroelectric mixed liquid crystal exhibits V-shapedelectrooptical response characteristics, and the liquid crystal in whichits driving voltage is about ±2.5 V (cell thickness is about 1 μm to 2μm) has also been found.

[0114] Here, FIG. 6 shows an example of characteristics of lighttransmittance of the thresholdless antiferroelectric mixed liquidcrystal showing the V-shaped electrooptical response to applied voltage.The vertical axis of the graph shown in FIG. 6 indicates thetransmittance (in arbitrary unit) and the horizontal axis indicates theapplied voltage. Incidentally, the transmission axis of a polarizingplate of a liquid crystal display device at an incident side is setalmost parallel to a normal direction of a smectic layer of thethresholdless antiferroelectric mixed liquid crystal which is almostcoincident with a rubbing direction of the liquid crystal displaydevice. The transmission axis of the polarizing plate at an outgoingside is set almost normal (crossed Nicols) to the transmission axis ofthe polarizing plate at the incident side.

[0115] As shown in FIG. 6, it is understood that when such athresholdless antiferroelectric mixed liquid crystal is used, lowvoltage driving and gradation display become possible.

[0116] In the case where such a low voltage driving thresholdlessantiferroelectric mixed liquid crystal is used for a liquid crystaldisplay device having an analog driver, it becomes possible to suppressthe source voltage of a sampling circuit of an image signal to, forexample, about 5 V to 8 V. Thus, the operation source voltage of thedriver can be lowered, and low power consumption and high reliability ofthe liquid crystal display device can be realized.

[0117] Also in the case where such a low voltage driving thresholdlessantiferroelectric mixed liquid crystal is used for a liquid crystaldisplay device having a digital driver, an output voltage of a D/Aconversion circuit can be lowered, so that the operation source voltageof the D/A conversion circuit can be lowered and the operation sourcevoltage of the driver can be made low. Thus, low power consumption andhigh reliability of the liquid crystal display device can be realized.

[0118] Thus, to use such a low voltage driving thresholdlessantiferroelectric mixed liquid crystal is also effective in the casewhere a TFT having an LDD region (low concentration impurity region)with a relatively small width (for example, 0 nm to 500 nm or 0 nm to200 nm) is used.

[0119] In general, the thresholdless antiferroelectric mixed liquidcrystal has large spontaneous polarization, and the dielectric constantof the liquid crystal itself is high. Thus, in the case where thethresholdless antiferroelectric mixed liquid crystal is used for aliquid crystal display device, it becomes necessary to providerelatively large holding capacitance for a pixel. Thus, it is preferableto use the thresholdless antiferroelectric mixed liquid crystal havingsmall spontaneous polarization. Besides, it is also permissible todesign such that a driving method of the liquid crystal display deviceis made linear sequential driving, so that a writing period (pixel feedperiod) of a gradation voltage to a pixel is prolonged and holdingcapacitance is compensated even if it is small.

[0120] Since low voltage driving can be realized by using such athresholdless antiferroelectric mixed liquid crystal, low powerconsumption of the liquid crystal display device can be realized.

[0121] Incidentally, as long as a liquid crystal has electroopticalcharacteristics as shown in FIG. 6, any liquid crystal can be used as adisplay medium of a liquid crystal display device of the presentinvention.

[0122] Embodiment 5

[0123] The CMOS circuit or pixel matrix circuit formed by implementingthe present invention can be used for a variety of electroopticaldevices (such as active matrix type liquid crystal display, activematrix type EL display, and active matrix type EC display). That is, thepresent invention can be implemented by any electronic equipmentequipped with these electrooptical devices as display media.

[0124] Such electronic equipments include a video camera, a digitalcamera, a (rear-type or front-type) projector, a head mount display(goggle type display), a car navigation system, a personal computer, anda portable information terminal (mobile computer, cellular phone orelectronic book, etc.). FIGS. 3A to 3F and 4A to 4D depict examples ofthese equipments.

[0125]FIG. 3A depicts a personal computer that is constituted by a mainbody 2001, an image input portion 2002, a display device 2003, and akeyboard 2004. The present invention can be applied to the image inputportion 2002, the display device 2003, and other signal controlcircuits.

[0126]FIG. 3B depicts a video camera that is constituted by a main body2101, a display device 2102, an audio input portion 2103, an operationswitch 2104, a battery 2105, and an image receiving portion 2106. Thepresent invention can be applied to the display device 2102, the audioinput portion 2103, and other signal control circuits.

[0127]FIG. 3C depicts a mobile computer that is constituted by a mainbody 2201, a camera unit 2202, an image receiving portion 2203, anoperation switch 2204, and a display device 2205. The present inventioncan be applied to the display device 2205 and other signal controlcircuits.

[0128]FIG. 3D depicts a goggle type display that is constituted by amain body 2301, a display device 2302, and an arm portion 2303. Thepresent invention can be applied to the display device 2302 and othersignal control circuits.

[0129]FIG. 3E depicts a player using a recording medium with a recordedprogram (hereinafter referred to as recording medium), that isconstituted by a main body 2401, a display device 2402, a speaker unit2403, a recording medium 2404, and an operation switch 2405.Incidentally, this apparatus uses a DVD (Digital Versatile Disc), a CD,and the like as the recording medium, and it is possible to appreciatemusic, to appreciate a movie, to play a game, and to perform theInternet. The present invention can be applied to the display device2402 and other signal control circuits.

[0130]FIG. 3F depicts a digital camera that is constituted by a mainbody 2501, a display device 2502, an eyepiece portion 2503, an operationswitch 2504, and an image receiving portion (not shown). The presentinvention can be applied to the display device 2502 and other signalcontrol circuits.

[0131]FIG. 4A depicts a front type projector that is constituted by adisplay device 2601, and a screen 2602. The present invention can beapplied to the display device 2601 and other signal control circuits.

[0132]FIG. 4B depicts a rear type projector that is constituted by amain body 2701, a display device 2702, a mirror 2703, and a screen 2704The present invention can be applied to the display device 2702 andother signal control circuits.

[0133]FIG. 4C shows an example of the structure of the display devices2601, 2702 shown in FIGS. 4A and 4B. The display devices 2601, 2702 eachare constituted by a light source optical system 2801, mirrors 2802,2804 to 2806, a dichroic mirror 2803, a prism 2807, a liquid crystaldisplay device 2808, a phase plate 2809, and a projection optical system2810. The projection optical system 2810 is constituted by an opticalsystem including a projection lens. Although this embodiment shows anexample of a three-plate type, it is not limited thereto, and a singleplate type may also be available, for instance. Further, optionally, anoptical system such as an optical lens, a film with a polarizationfunction, a film controlling the phase difference, and an IR film may bedisposed at the light path indicated by an arrow in FIG. 4C.

[0134]FIG. 4D shows an example of the structure of the light sourceoptical system 2801 of FIG. 4C. In this embodiment, the light sourceoptical system 2801 is constituted by a reflector 2811, light sources2812, 2813, 2814, a polarization conversion component 2815, and acondenser 2816. Incidentally, the light source optical system shown inFIG. 4D is merely an example but is not limited thereto. For example,optionally, an optical system such as such as an optical lens, a filmwith a polarization function, a film controlling the phase difference,and an IR film may be disposed.

[0135] As set forth above, the scope of application of the presentinvention is extremely wide and the present invention can be applied toelectronic equipments of any field. Moreover, the electronic equipmentsof this embodiment can be realized even if a structure of anycombination of the embodiments 1 to 5 is used.

What is claimed is:
 1. A method of manufacturing a semiconductor devicecomprising the steps of: forming a semiconductor film comprising siliconover a substrate; irradiating said semiconductor film with laser lightin air for crystallizing said semiconductor film; removing an oxide filmfrom a surface of the semiconductor film by etching after theirradiation of the laser light; and leveling the surface of thesemiconductor film by heating after removing said oxide film.
 2. Amethod of manufacturing a semiconductor device comprising the steps of:forming a semiconductor film comprising silicon over a substrate;irradiating said semiconductor film with laser light in air forcrystallizing said semiconductor film; removing an oxide film from asurface of the semiconductor film by etching after the irradiation ofthe laser light; and leveling the surface of the semiconductor film byheating in a reducing atmosphere after removing said oxide film.
 3. Amethod of manufacturing a semiconductor device comprising the steps of:forming a semiconductor film comprising silicon over a substrate;irradiating said semiconductor film with laser light in air forcrystallizing said semiconductor film; removing an oxide film from asurface of the semiconductor film by etching after the irradiation ofthe laser light; and leveling the surface of the semiconductor film byheating in an inert gas after removing said oxide film.
 4. A method ofmanufacturing a semiconductor device comprising the steps of: forming asemiconductor film comprising silicon over a substrate; irradiating saidsemiconductor film with laser light in air for crystallizing saidsemiconductor film; removing an oxide film from a surface of thesemiconductor film by etching after the irradiation of the laser light;and leveling the surface of the semiconductor film by heating in anatmosphere after removing said oxide film, a concentration of oxygen ora oxygen compound contained in said atmosphere is 10 ppm or less.
 5. Amethod of manufacturing a semiconductor device comprising the steps of:forming a semiconductor film comprising silicon over a substrate;irradiating said semiconductor film with laser light in air forcrystallizing said semiconductor film; removing an oxide film from asurface of the semiconductor film by etching after the irradiation ofthe laser light; and leveling the surface of the semiconductor film byheating in a reducing atmosphere after removing said oxide film, aconcentration of oxygen or a oxygen compound contained in said reducingatmosphere is 10 ppm or less.
 6. A method of manufacturing asemiconductor device comprising the steps of: forming a semiconductorfilm comprising silicon over a substrate; irradiating said semiconductorfilm with laser light in air for crystallizing said semiconductor film;removing an oxide film from a surface of the semiconductor film byetching after the irradiation of the laser light; and leveling thesurface of the semiconductor film by heating in an inert gas afterremoving said oxide film, a concentration of oxygen or a oxygen compoundcontained in said inert gas is 10 ppm or less.
 7. A method ofmanufacturing a semiconductor device comprising the steps of: forming asemiconductor film comprising silicon over a substrate; irradiating saidsemiconductor film with laser light in air for crystallizing saidsemiconductor film; treating a surface of the semiconductor film with ahydrofluoric acid after the irradiation of the laser light; and levelingthe surface of the semiconductor film by heating after the treatmentwith said hydrofluoric acid.
 8. A method of manufacturing asemiconductor device comprising the steps of: forming a semiconductorfilm comprising silicon over a substrate; irradiating said semiconductorfilm with laser light in air for crystallizing said semiconductor film;treating a surface of the semiconductor film with a hydrofluoric acidafter the irradiation of the laser light; and leveling the surface ofthe semiconductor film by heating after the treatment with saidhydrofluoric acid in a reducing atmosphere.
 9. A method of manufacturinga semiconductor device comprising the steps of: forming a semiconductorfilm comprising silicon over a substrate; irradiating said semiconductorfilm with laser light in air for crystallizing said semiconductor film;treating a surface of the semiconductor film with a hydrofluoric acidafter the irradiation of the laser light; and leveling the surface ofthe semiconductor film by heating after the treatment with saidhydrofluoric acid in an inert gas.
 10. A method of manufacturing asemiconductor device comprising the steps of: forming a semiconductorfilm comprising silicon over a substrate; irradiating said semiconductorfilm with laser light in air for crystallizing said semiconductor film;treating a surface of the semiconductor film with a hydrofluoric acidafter the irradiation of the laser light; and leveling the surface ofthe semiconductor film by heating after the treatment with saidhydrofluoric acid in an atmosphere, a concentration of oxygen or aoxygen compound contained in said atmosphere is 10 ppm or less.
 11. Amethod of manufacturing a semiconductor device comprising the steps of:forming a semiconductor film comprising silicon over a substrate;irradiating said semiconductor film with laser light in air forcrystallizing said semiconductor film; treating a surface of thesemiconductor film with a hydrofluoric acid after the irradiation of thelaser light; and leveling the surface of the semiconductor film byheating after the treatment with said hydrofluoric acid in a reducingatmosphere, a concentration of oxygen or a oxygen compound contained insaid reducing atmosphere is 10 ppm or less.
 12. A method ofmanufacturing a semiconductor device comprising the steps of: forming asemiconductor film comprising silicon over a substrate; irradiating saidsemiconductor film with laser light in air for crystallizing saidsemiconductor film; treating a surface of the semiconductor film with ahydrofluoric acid after the irradiation of the laser light; and levelingthe surface of the semiconductor film by heating after the treatmentwith said hydrofluoric acid in an inert gas, a concentration of oxygenor a oxygen compound contained in said inert gas is 10 ppm or less. 13.A method of manufacturing a semiconductor device according to any one ofclaims 1-12, wherein the step of leveling the surface of saidsemiconductor film is conducted by furnace annealing.
 14. A method ofmanufacturing a semiconductor device according to any one of claims1-12, wherein the step of leveling the surface of said semiconductorfilm is conducted between 900 and 1200° C.
 15. A method of manufacturinga semiconductor device according to any one of claims 3, 6, 9, and 12,wherein said inert gas is nitrogen.
 16. A method of manufacturing asemiconductor device according to any one of claims 2, 5, 8, and 11,wherein said reducing atmosphere comprises hydrogen.
 17. A method ofmanufacturing a semiconductor device according to any one of claims1-12, further comprising a step of treating a surface of thesemiconductor film with a buffered hydrofluoric acid before theirradiation of the laser light.
 18. A method of manufacturing asemiconductor device according to any one of claims 1-12, wherein saidsemiconductor device is one selected from the group consisting of apersonal computer, a video camera, a goggle-type display, a digitalcamera, and a projector.